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Thread Like Summary
Total Likes: 3
Original Post (Thread Starter)
by Mark Parker
Mark Parker
67hp is what Honda claimed for their newly introduced CB750 OHC 4. It's quite possible at some stage Honda actually achieved that power on their dyno. Rear wheel dyno graphs of CB750s sold to the public proved to be shy of that output even when converting for the losses through the drive train. One CB750 tested in the US delivered 54rwhp @ 8,000rpm indicating around 61.5hp at the crankshaft.

What interests me is the fact that the BSA factory chose not to machine A65 inlet ports to the actual design drawing, ever, and when Umbeslade Hall technicians actually machined the inlet ports to the design drawings their motor made 8hp more on their dyno than production line units. This was an oil in frame photo shoot Firebird built in 1970. Not only was the engine more powerful it was also more responsive. The only difference was the machining of the cylinder head.

I'm curious what the result would be to machining the A65 inlet ports to one of the most efficient two valve port shapes known today. To do this I've enlarged the port slightly to 32mm and used a 42mm inlet valve from MAP Cycles. I'll also use slightly O/Size exhaust valves as well, though its probably unnecessary. So the head is ported and on my somewhat home made flow bench is indicating an excellent gain in flow 160cfm @ 28"W @.385"of lift compared to the factory heads 109cfm.

Fitting a round-slide carb restricts flow especially at 30-32mm bore. A 32mm Mikuni with long bellmouth reduced flow to 135.2cfm, with the bellmouth removed it was better at 139.17cfm, blending the carb intake enabled 145.5cfm.

Using Wallace Racing's on line hp calculator which bases its figures on intake port flow a 40cubic inch twin flowing 109cfm can produce 52hp at the crank (similar to BSAs claimed output) or 45.7rwhp approx. though like the XS650 Yamaha stock A65s are usually around 43-44rwhp. Calculating for 145cfm gives a estimate of 69hp at the crank, if it could achieve that it would translate to about 60.72rwhp.

So I'm interested to see if an otherwise stock A65 fitted with this head and 32mm carbs could equal or better the CB750s hp claim.

The modified carb:

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The new port shape:

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Not the actual head but one just like it.

[Linked Image from]

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Liked Replies
by Mark Parker
Mark Parker
It's not particularly hard to do the ports by hand. BSA could have done the ports at least to how they were designed rather than short cutting at the factory, it may have been saving them a little money and keep the A65 on parity with the Triumph twin which they also owned. In 1970 they knew the difference between the heads but didn't rectify the production, they could have had 8 more hp in 1971. In their day these bikes were among the fastest most powerful on the road, but they were about to be displaced, or by 1971 they were being displaced by the 750s and the power bikes from Japan.

It was hardly a time to throw away easy hp in what was a full-scale hp race. You can see this when the mighty Z1 Kawasaki turned up, it was called the King. It was developed as a 750 but Honda released the CB750 4, getting the jump. Kawasaki held off releasing their bike till they increased the displacement to 903cc the reason they did that was to sell bikes, they understood the power of power. Honda claimed 67hp Kawasaki trounced it with 82hp and 54.2ftlb, though it was heavy at 542lb. For perspective my 883cc A65 weighs around 360lb and produces 97hp and 71ftlb it also has a very wide power spread. It uses a bigger version of the oval port above, so I'm interested to see how that translates onto an otherwise stock engine.

BSA already knew how to fix all the problems with the A65, roller mains both sides would have reversed market resistance, 71 or earlier they had excellent steel capped rods, high volume oil pumps and they had the A70 showing great potential. But they sank money and effort into things like the Ariel 3. And also the 350s that were a good idea though eventually scrapped because of a shot to the foot.

The 350 was a missed opportunity, they could not get good power from it because the head design was poor, DH had made an error, easy to rectify with a new casting, but management told him no. So 34hp was the best they could ever get. The 350 Honda was already in the market with similar power, the BSA 350 could and should have made 45+hp, and it would have been a winner, it could then be more expensive than the Honda because it gave more, it had the advantage with a state of the art frame and running gear already. A company like Kawasaki would have made sure they had what they needed to get the power necessary. So rather than spend massive effort and money fruitlessly down these paths, BSA could have done three things, fixed the A65 engine completely, and at the same time, because its basically the same thing, developed the A70And the third, built at least a limited number of Rob North style Rocket threes, the frames were state of the art and exotic, cheaper to manufacture than the std frames. The threes also work extremely well at 850cc if they wanted to respond to the Z1 and Ducati's SS750 when they came along.
1 member likes this
by NickL
Originally Posted by DavidP
I hear it all the time, "Car engines used bushings on the crank."
The only one with which I am familiar enough to comment is the air-cooled VW. Yes, it used a bushed crank. No, it did not feed oil through the bush! It had oil feeds at two places along the crank. And, it was a wet-sump engine. And, if you wanted to rev it past about 4500rpm you had to dowel the crank so it wouldn't beat the bearings to death. Oddly enough, those engines could be built to deliver twice the HP and still be reliable.
BTW: My mid-71 A65 came with the crap-metal oil pump and alloy rod caps. Too little, too late.
There's a tale from Healy in Vintage Bike. They disassembled the first Trident they got at his shop and found casting sand inside the engine. So much for the artisans at Small Heath.

Many blokes are riding their standard setup a65's quite happily, me included, with alley rods and a standard timing side bush. I certainly don't 'baby' my road bikes and i know
the racing outfit i prepped last year (also standard) gets used to 7-7500 often. I really think you suffer from a phobia about beezers. As i've said before, why not just sell it? (and buy another vw)
The artisans at beezer were renowned for 'doing the dirty' on triumphs wherever possible, human nature towards the competition i suppose. Chances are, the blokes at triumph would have done the same.
The bearing used on the a65 timing side is very similar to a car main with the exception of it not being split. Only aftermarket bearings were in fact a bronze bush. Either type work well if
installed correctly. Look at the racing history of bsa during the '50's when they produced the goldie if you need proof they could get a proper production bike to go, it won everything.
1 member likes this
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